1. Field of the Invention
The present invention relates to a magnetic recording medium and a manufacturing method thereof and more particularly to a magnetic recording medium of a multilayer coating system excellent in running durability and a manufacturing method thereof.
2. Description of the Related Art
Magnetic recording apparatus attendant, for example, on computers, video or audio apparatus have been developed in recent years more and more regarding reduction of size and weight, long time operation and digitalized recording, so that recording at higher density has been demanded more also to magnetic recording media used for such high performance magnetic recording apparatus. As the magnetic recording media, coated type media have been used predominantly. The coated type magnetic recording medium uses a magnetic recording layer formed by dispersing and kneading a magnetic powder, an organic binder and various kinds of additives in an organic solvent to form a magentic tape, and coating, drying and curing the magnetic coating material on a non-magnetic support.
Also in a coated type magnetic recording medium, a demand for recording at higher density and recording at shorter wavelength has been increased more and more and a large capacity floppy disk (FD) of about 100 MB has been put to practiced use in the field of 3.5" class FD, for instance, and development has also been made to FD of further larger capacity.
One of methods for improving electromagnetic conversion characteristics of a magnetic recording medium in a high density recording region, reduction of the film thickness for the magnetic recording layer is mentioned. By decreasing the thickness of the magnetic recording layer, demagnetizing field is decreased to reduce self-demagnetization loss upon recording. Further, thickness loss due to phasal displacement is decreased upon playback to improve electro-magnetics conversion characteristic in a short wavelength region. Excellent characteristics are obtained, particularly, in FD also for overwriting Characteristics, peak characteristics and peak semi-value width characteristics by making the magnetic recording layer into a thin films.
However, one of the problems caused by reducing the thickness of the magnetic recording layer is increase of spacing loss due to unevenness on the surface of the magnetic recording layer. That is, as the thickness of the magnetic recording layer is reduced, unevenness on the underlying non-magnetic support surface is reflected just as it is on the surface property of the magnetic recording layer. Accordingly, if the surface of the non-magnetic support is coarse, the surface of the magnetic recording layer is also made coarse and, as a result, it induces deterioration in the electromagnetic conversion characteristics or drop out characteristics of the magnetic recording medium.
Further, in the large capacity FD described previously, high speed rotation is required for improving the data transmission rate. Therefore, the FD is put under severe conditions relative to the sliding movement with a magnetic head, which gives an negative factor relative to the sliding movement with the magnetic heads in conjunction with the reduced thickness of the magnetic recording layer.
As described above, for attaining the reduction of the film thickness in the magnetic recording layer required in view of the electro-magnetic conversion characteristics, it is necessary to attain a high level surface design of the magnetic recording medium and excellent durability to a high speed sliding system together.
In order to cope with this demand, a multilayer coating system of interposing a non-magnetic under layer which is relatively thick and excellent in surface property between a non-magnetic support and a magnetic recording layer has been proposed in the coated type magnetic recording media. In the multilayer coating system, the surface shape of the magnetic recording layer can be designed freely not depending on the surface shape of the non-magnetic under layer. Further, it is also possible to improve the durability of the magnetic recording medium by providing various functions to the non-magnetic under layer. That is, demands for the high density recording and the durability can be made compatible by the multilayer coated type magnetic recording medium.
For preparing a multilayer coated type magnetic recording medium, a non-magnetic under layer material prepared by dispersing a non-magnetic powder and an organic binder into a solvent is coated on a non-magnetic support to form a non-magnetic under layer. Subsequently, a magnetic recording layer coating material prepared by dispersing a magnetic powder and an organic binder into a solvent is coated on the non-magnetic under layer described above to form a magnetic recording layer. In the case of a FD, the non-magnetic under layer and the magnetic recording layer are formed on both surfaces of the non-magnetic support. Further, as another example of the multilayer coated type magnetic recording medium, a magnetic under layer may be formed with a magnetic powder different from the magnetic powder for the upper layer, and a magnetic recording layer is disposed thereon.
Subsequently, calendering, heat curing and like other treatment are applied and then the laminate is put to a slitting step into a tape shape in the case of a magnetic recording tape or put to a punching step into a disk-like shape in the case of a FD, which is then contained in a cassette or a shell to complete a desired magnetic recording medium.
By the way, if fine dusts are deposited on the magnetic recording layer, drop-out (missing pulse), injury in the magnetic recording layer and, further, injury on the truck surface of a magnetic head may sometimes occur.
Therefore, also in FD, various attempts have been made for the shell structure in order to prevent intrusion of dusts into the shell.
Further, a non-woven fabric of long fibers referred to as a liner is usually contained so as to put a magnetic disk therebetween. Since the magnetic disk rotates in loose contact with the liner, the liner sweeps off dusts on the magnetic disk to keep the surface of the magnetic disk clean.
However, direct contact of the magnetic disk surface with the liner induces an additional problem. For instance, there is a phenomenon that a surface lubricant on the surface of the magnetic recording layer indispensable to the running durability is migrated to or scraped by the liner to worsen the running property. Particularly, in a large capacity FD requiring high speed rotation, depletion of the surface lubricant is a problem that has to be solved by all means.